recycling

Review

Recycling of Polymer-Based Multilayer Packaging:A Review

Katharina Kaiser 1,2,*, Markus Schmid 1,2 ID and Martin Schlummer 2 ID

1 Technical University of Munich, TUM School of Life Sciences Weihenstephan, Weihenstephaner Steig 22,85354 Freising, Germany; markus.schmid@ivv.fraunhofer.de

2 Fraunhofer Institute for Process Engineering and Packaging IVV, Giggenhauser Strasse 35, 85354 Freising,Germany; martin.schlummer@ivv.fraunhofer.de

* Correspondence: katharina.kaiser@ivv.fraunhofer.de; Tel.: +49-8161-491-466

Received: 27 October 2017; Accepted: 19 December 2017; Published: 22 December 2017

Abstract: Polymer-based multilayer packaging materials are commonly used in order to combinethe respective performance of different polymers. By this approach, the tailored functionality ofpackaging concepts is created to sufficiently protect sensitive food products and thus obtain extendedshelf life. However, because of their poor recyclability, most multilayers are usually incineratedor landfilled, counteracting the efforts towards a circular economy and crude oil independency.This review depicts the current state of the European multilayer packaging market and sketchesthe current end-of-life situation of postconsumer multilayer packaging waste in Germany. In themain section, a general overview of the state of research about material recycling of differentmultilayer packaging systems is provided. It is divided into two subsections, whereby one describesmethods to achieve a separation of the different components, either by delamination or the selectivedissolution–reprecipitation technique, and the other describes methods to achieve recycling bycompatibilization of nonmiscible polymer types. While compatibilization methods and the techniqueof dissolution–reprecipitation are already extensively studied, the delamination of packaging has notbeen investigated systematically. All the presented options are able to recycle multilayer packaging,but also have drawbacks like a limited scope or a high expenditure of energy.

Keywords: multilayer packaging; recycling; delamination; selective dissolution; compatibilization

1. Introduction

Because of the considerably high amounts of packaging waste produced in their daily lives,most consumers perceive plastic packaging in a negative way by. This negative judgement, however,disregards the positive impact of packaging on sustainability by protection of the packaged goods.The resources contained in those goods and the amount of water, farmland, and energy that werenecessary for their production makes them valuable products, worthy of protection. Protection can beprovided directly by preventing the good from contamination and indirectly by extending its shelflife. In addition to the protection, transportability and storage capability are also primary functionsof packaging. Secondary functions, like the communication of information and the promotion of thearticle, are also important aspects. Since the demand of both primary and secondary functions isgrowing, the complexity of packaging is increasing [1,2].

Initially, packaging consisted of a single material. In many cases, this concept works fine, such asfor glass jars, polyethylene terephthalate (PET) soda bottles, or metal cans. However, each of thesematerials has a limitation that inhibits its broader utilization: glass is heavy and fragile, PET does notprovide a sufficient oxygen barrier for many products, and metal is not transparent. By combiningdifferent materials, ideal packaging concepts for most packaging requirements can be tailored [3].While for some kinds of multilayer packaging, like LPBs (liquid packaging boards), an end-of-life

Recycling 2018, 3, 1; doi:10.3390/recycling3010001 www.mdpi.com/journal/recycling

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recycling option is already established on the market [4], for polymer–polymer multilayer materialsno industry solution has been available until now. For the recycling of multilayer postindustrialwaste [4,5], a delamination process has been proposed.

In a previous review from 1996, Hensaw et al. [6] reviewed the general recycling options forpolymer composite materials, comprising pyrolysis, hydrolysis, energy recovery, and compression–injection molding. In 2011, Jesmy et al. [7] gave an overview of the recycling of polymer blends with theexclusive description of recycling by extrusion and compatibilizer-aided blending, with only one smallabstract dealing with packaging materials. Hamad et al. [8] described methods to recycle polymericmaterials by reprocessing and compounding, as well as by polymer degradation. The advantages andlimitations of the dissolution—reprecipitation technique were discussed in a conference abstract byAchilias et al. [9]. In 2017, Ragaert et al. [10] published a review that provides a detailed descriptionof the current pathways for the recycling of solid plastic waste, via both mechanical and chemicalrecycling. As a promising option for multilayer packaging recycling, pyrolytic decomposition wasmentioned. In the eyes of the authors, the complexity of the packaging systems is an argument againstmechanical recycling.

Nonetheless, several approaches to the mechanical recycling of multilayers are described in theliterature. In this study, an overview of those solutions, approaches, and ideas concerning the recyclingof multilayered packaging systems is provided. Consideration was hereby given to multilayer systemswith at least two materials, of which at least one is made of plastic. Other materials can be metalslike aluminum or a fibrous material like cardboard. Those strategies, in general, can be divided intotwo categories. The first category includes methods according to which recycling is performed bythe separation of the different components of the multilayer, either by the dissolution–reprecipitationtechnique or the delamination of the multilayer. The delamination can be performed physically(e.g., by selective dissolution), chemically (e.g., by the reactive removal of an interlayer), or mechanically.The second strategy comprises the joint processing of the constituents of the multilayer packaging,whereby the use of additives is often necessary. Additionally, an overview of the packaging marketand the state of the art of the end-of-life treatment of multilayer packaging waste is given. This reviewis the first article that provides an overview and detailed description of the different techniques ofmultilayer packaging recycling. Additionally, these techniques are discussed and evaluated critically.

2. Packaging and Current End-of-Life Situation

2.1. Packaging

2.1.1. Plastic Market and Applications

In Europe, 49 million tons of oil and gas were used for the production of plastic in 2015,constituting 4 to 6% of the total amount. About 40% thereof was used for packaging [11]. Polyolefins(PO) make up the biggest share thereof, with low-density polyethene (LDPE) being the most prominent,followed by polypropene (PP), high-density polyethene (HDPE), and polyethylene terephthalate(PET). Smaller shares are contributed by polyvinyl chloride (PVC), polystyrene (PS), polyamide (PA),and other plastics (cf. Figure 1) [11,12].

For multilayer packaging, the typical polymers listed in Figure 1 as well as more special polymerswith useful functions are generally used [13]. One reason for the growing number of multilayerpackaging is that costs can be reduced by adding an additional number of layers and thus decreasingthe amount of material, in comparison to what would be needed to make a single layer performthe same function. More often, however, the multilayer structure allows the package to perform acombination of functions that is not possible with a single layer [1,14].

Recycling 2018, 3, 1 3 of 26Recycling 2017, 2, 25 3 of 25

Figure 1. Fields of application of plastic materials and polymer types predominately used in

packaging. The respective share of the different polymers is derived from a graphic illustration of the

polymeric composition of plastic packaging in Europe [11]. Abbreviations used: LDPE (low-density

polyethene), PP (polypropene), HDPE (high-density polyethene), PET (polyethylene terephthalate),

PVC (polyvinyl chloride), PS (polystyrene), PA (polyamide),

2.1.2. Flexible Plastic Packaging Market

A two-layer packaging obviously consists of an inner side, facing the product, and an outer layer.

Usually, in this case, the inner layer provides sealability, and the outer layer resistance against

abrasion, printability, or barrier. Common sealant layers provide low-temperature sealability for fast

packing line speeds and may also contribute to barrier performance. Ethylene-vinyl acetate (EVA),

LDPE, linear-low-density polyethene (LLDPE), PE plastomers, and ionomers are standardly used,

among which ionomers additionally provide exceptional oil and grease resistance [3].

Because those sealants often lack stiffness, structural integrity, and abrasion resistance, an outer

layer, which also may bring an additional functionality, is used. HDPE provides a moisture barrier

and is often used in packaging dry foods such as cereal and crackers. PA may be used where

exceptional mechanical resistance is needed, such as in brick packages for cheese. Oriented PET films

provide an excellent surface for printing. Paper is a low-cost substrate often used in pouches and bags

[3].

A three-layer packaging provides even greater flexibility in packaging design. Sealability is still

the primary function of the inner layer, whereas the outer layer is supposed to provide abrasion

resistance, heat resistance (during sealing), stiffness, structural integrity, a surface for printing, and

in some cases, a moisture barrier. The additional center layer can provide a variety of functions. An

oxygen barrier may be provided, e.g., by EVOH or a metallization layer. In some cases, an adhesive

may be needed in the center layer to allow for the lamination of two dissimilar materials. In many

cases, reverse printing is applied, which means that films are printed and laminated to a sealant layer

with the print side facing inward to ensure protection from the outside by the film layer. In food

packaging, also a layer of recycled material is allowed to be incorporated into the center layer for

some applications [3,14,15] As specified in Regulation (EU) No 10/2011, recycled plastics from

commingled postconsumer waste should not be in direct contact with food. Nonetheless, they can be

integrated in an inner layer of the packaging if they are separated by a functional barrier that avoids

the transfer of non-authorized substances from the recycled layer to the foodstuff. To avoid set-off

migration effects through the nonfood contact side of the packaging to the food contact side of other

packaging during the storage of stackable packaging, symmetrical multilayer structures with

Others22%

Building & Construction

19%

Electrical & Electronic

6%

Automotive9%

Agriculture3%

Packaging40%

LDPE

PP

HDPE

PET

PS

PVC

other plastics

Figure 1. Fields of application of plastic materials and polymer types predominately used in packaging.The respective share of the different polymers is derived from a graphic illustration of the polymericcomposition of plastic packaging in Europe [11]. Abbreviations used: LDPE (low-density polyethene),PP (polypropene), HDPE (high-density polyethene), PET (polyethylene terephthalate), PVC (polyvinylchloride), PS (polystyrene), PA (polyamide).

2.1.2. Flexible Plastic Packaging Market

A two-layer packaging obviously consists of an inner side, facing the product, and an outer layer.Usually, in this case, the inner layer provides sealability, and the outer layer resistance against abrasion,printability, or barrier. Common sealant layers provide low-temperature sealability for fast packingline speeds and may also contribute to barrier performance. Ethylene-vinyl acetate (EVA), LDPE,linear-low-density polyethene (LLDPE), PE plastomers, and ionomers are standardly used, amongwhich ionomers additionally provide exceptional oil and grease resistance [3].

Because those sealants often lack stiffness, structural integrity, and abrasion resistance, an outerlayer, which also may bring an additional functionality, is used. HDPE provides a moisture barrier andis often used in packaging dry foods such as cereal and crackers. PA may be used where exceptionalmechanical resistance is needed, such as in brick packages for cheese. Oriented PET films provide anexcellent surface for printing. Paper is a low-cost substrate often used in pouches and bags [3].

A three-layer packaging provides even greater flexibility in packaging design. Sealability is still theprimary function of the inner layer, whereas the outer layer is supposed to provide abrasion resistance,heat resistance (during sealing), stiffness, structural integrity, a surface for printing, and in some cases,a moisture barrier. The additional center layer can provide a variety of functions. An oxygen barriermay be provided, e.g., by EVOH or a metallization layer. In some cases, an adhesive may be needed inthe center layer to allow for the lamination of two dissimilar materials. In many cases, reverse printingis applied, which means that films are printed and laminated to a sealant layer with the print sidefacing inward to ensure protection from the outside by the film layer. In food packaging, also a layerof recycled material is allowed to be incorporated into the center layer for some applications [3,14,15]As specified in Regulation (EU) No 10/2011, recycled plastics from commingled postconsumer wasteshould not be in direct contact with food. Nonetheless, they can be integrated in an inner layer ofthe packaging if they are separated by a functional barrier that avoids the transfer of non-authorizedsubstances from the recycled layer to the foodstuff. To avoid set-off migration effects through thenonfood contact side of the packaging to the food contact side of other packaging during the storageof stackable packaging, symmetrical multilayer structures with functional barriers at both sides of therecycled layer can also be developed. For nonfood packaging, no restriction concerning the contactwith the filling material exists [16,17]).

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In extended concepts like five-layered films, the centered layer is often surrounded by twoadhesive layers that enable adhesion to the outer and inner layer. For example, this structure iscommon for barrier films with PA-6 or EVOH, since these polymers do not adhere to most sealantsand outer layers [3]. An overview of commonly used materials and their respective functions is shownin Table 1.

Table 1. Overview of common functional layers [3,14,18]. Abbreviations used: BOPA (biaxially orientedpolyamide), EVA (polyethylene-vinyl acetate), OPA (oriented polyamide), OPET (oriented polyethyleneterephthalate), OPP (oriented polypropene), PVDC (polyvinylidene chloride), PVOH (polyvinylalcohol), PE (polyethene), LD (low-density), LLD (linear low-density), HD (high-density).

MechanicalStability Oxygen Barrier Moisture

BarrierLight

Barrier Tie Layers Sealant

HDPE EVOH PE (LD,LLD, HD) aluminum polyurethanes LLDPE

PP, OPP PVDC PP, OPP TiO2 filledpolymers

Acid/anhydridegrafted polyolefins LDPE

OPET polyamides(nylon, BOPA) EVA EVA

PS polyesters, OPET ionomers ionomers

Paper coatings (SiOx, Al2O3,PVOH, nano particles) PVDC PP, OPP

aluminum PA, OPA

PET, OPET

The GVM (Association of Packaging Market Research) [13] divides multilayer packaging filmsinto five categories, whereby only multilayered packaging with a polymer content higher than 50%,and packaging without paper are considered:

• flexible plastic and plastic composites without barrier layer (“simple multilayers”)• flexible plastic and plastic composites with an organic barrier layer• flexible metallized plastic and plastic composites films with coatings based on AlOx or

SiOx, respectively• thermoformed plastic and plastic composites• plastic and plastic composites with Al foil.

These five categories made up 17.7 billion m2 (approximately 1.89 Mt; the number of ~1.89 Mtwas derived by calculation of the ratio of the amount of multilayer packaging in Germany in 2009 tothe area of multilayer packaging in Germany in 2009 [1]. By using this ratio, the mass of multilayerpackaging was derived from the area of multilayer packaging in 2016 in Europe [13]. Thus, the numberof 1.89 Mt is an approximation based on the assumption that the composition of multilayer packagingwas the same in Germany in 2009 as in Europe in 2016. The respective shares of the different categoriesare given in Table 2, along with the use of one example of the corresponding category [13].

The sum of the five most important material combinations in each of the five categories givenin Table 2 makes up 56.3 % m2 of plastic-based flexible packaging. The remaining 43.7 % m2 consistsof a large number of material combinations, whereof simple PET–PO multilayers and thermoformedPET–PO multilayers still represent an important larger proportion [13].

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Table 2. Quantities of the five flexible plastic-based multilayer packaging categories and, in each case,the most important example is given in millions (mio.) m2 and in percent m2. The percentage refers tothe sum of packaging used in these categories in Europe in 2016 [13]. PET-BO is the abbreviation forbiaxially-oriented PET.

Group Materials Mio. m2 Film Share in m2-%

Simple multilayers total 6565.6 37.1Thereof: PA/polyolefin 2710.2 15.3

Packaging with organic barriers total 4499.9 25.4Thereof: EVOH 4304.7 24.3

Thermoformed packaging total 2482.1 14.0Thereof: PA/polyolefin 756,3 4.27

Packaging with inorganic barriers total 2996.1 16.9Thereof: PET-BO/layer/polyolefin 1362.9 7.7

Packaging with Al-foil total 1153.1 6.5Thereof: PET/Al/plastic 829.1 4.7

17,696.9 100

2.1.3. Composite Packaging Containing Paper and Plastic

LPBs (liquid packaging board) consists of about 21% PE, 4% aluminum, and about 75% cardboard.The cardboard layer is supposed to provide stability, the PE layers provide protection against moisturefrom the outer and the inner side of the packaging, while the aluminum acts as a gas barrier. In 2016,178 kt of LPBs were used in Germany [19,20].

2.1.4. Trend towards Avoidance of Multilayer Packaging

Currently, an opposite trend towards the broader use of multilayers can be observed. Becauseof the more environmentally friendly image of monomaterial packaging, ambitions towards thesubstitution of multilayer packaging by monolayers can be observed. For example, the RecycleReadyTechnology of DOW® (Midland, MI, USA) allows for the substitution of heterogeneous multilayerpackaging, e.g., containing PET and PE, by an all-polyolefine packaging containing a barrieradhesive [21]. Based on a combination of the proprietary Borstar®-Technology for bimodal PE andthe machine direction-oriented (MDO) processing technology, Borealis developed a full PE laminateas an alternative to multilayered structures [22]. In general, however, the substitution of multilayerpackaging systems should only be carried out if the protection of the product can be equally assured.

2.2. End-of-Life Treatment of Multilayer Packaging

The end-of-life treatment of postconsumer packaging waste comprises three steps:

• collection• sorting• reprocessing

A proper collection is a prerequisite for an effective sorting and collection. However, since itis a question of logistics and not a technical problem, it does not lie within the scope of this review.An overview of the different collection systems is given in [23].

In this section, the state of the art of multilayer packaging sorting and reprocessing is summarized.Subsequently, in the next section, the state of science of multilayer reprocessing is described in detail.

The sorting process of postconsumer waste is essential to generate a decent amount and qualityof recycling material. In Figure 2, a sorting scheme of the lightweight packaging fraction is shown. It isa simplified example of the structure of a sorting plant intended to sort the “yellow sack” producedwithin the “Duales System” in Germany. The input contains postconsumer packaging made of plastic,

Recycling 2018, 3, 1 6 of 26

metals, and composite packaging. Material recovery facilities in other European countries like Belgiumor the Netherlands follow similar standards [24].Recycling 2017, 2, 25 6 of 25

Container/bag/bring-system

Classification

Wind-sifting

Magnetic separator

NIR-LPB

Eddy-current separator

NIR-mixed plastics

NIR-paper and board

Bag opener

Wind-sifting

NIR-paper/tetra/PET

NIR-LPB

NIR-standard polymer

Modules of recycling

Input

>220mm<20mmResidue

ResiduePaperand

boardPEPPPSPET

Mixedplastics

rigid

Nonferrousmetals

LPBFerrous metals

Mixedplastic

filmFilms

Collection

Sorting

Recovery

Figure 2. Scheme of a standard state-of-the-art sorting plant. Adapted from [20]. NIR is the

abbreviation for near infrared.

Thin-walled, flexible packaging items enter the film fraction and the mixed plastic films by

classification and wind sifting. Since the film fraction should only contain films bigger than DIN A4,

the amount of multilayer packaging contained in this fraction is not expected to be significantly high.

The mixed plastic films, instead, contain flexible packaging sized smaller than DIN A4 [25]. Here, a

higher amount of multilayer packaging is expected to be contained.

Downstream in the process, LPBs and plastic-coated cardboard packaging are identified by NIR

reflection measurements by means of a specific spectrum and separated from the other materials by

using pulses of compressed air. LPB is the only kind of multilayered packaging that is assigned to its

own sorting fraction [24].

Downstream, aluminum is removed from the main material flow by an eddy current separator

which sorts by electric conductivity. In Germany, unlike other European countries, polymer-based

packaging coated with aluminum is assigned to this aluminum fraction. Depending on the structure

of the multilayer, this separation step works more or less efficiently. Relatively low aluminum content

in combination with high polymer content can prevent aluminum from being separated [24].

Dimensionally stable plastics can be sorted by an NIR separator. Stable multilayer packaging

should be sorted into the rigid mixed plastics or remain in the residue.

Following sorting, the presorted polymers have to be recycled. This reprocessing of the sorted

polymers can be done in three ways:

Figure 2. Scheme of a standard state-of-the-art sorting plant. Adapted from [20]. NIR is the abbreviationfor near infrared.

Thin-walled, flexible packaging items enter the film fraction and the mixed plastic films byclassification and wind sifting. Since the film fraction should only contain films bigger than DIN A4,the amount of multilayer packaging contained in this fraction is not expected to be significantly high.The mixed plastic films, instead, contain flexible packaging sized smaller than DIN A4 [25]. Here,a higher amount of multilayer packaging is expected to be contained.

Downstream in the process, LPBs and plastic-coated cardboard packaging are identified by NIRreflection measurements by means of a specific spectrum and separated from the other materials byusing pulses of compressed air. LPB is the only kind of multilayered packaging that is assigned to itsown sorting fraction [24].

Downstream, aluminum is removed from the main material flow by an eddy current separatorwhich sorts by electric conductivity. In Germany, unlike other European countries, polymer-basedpackaging coated with aluminum is assigned to this aluminum fraction. Depending on the structure

Recycling 2018, 3, 1 7 of 26

of the multilayer, this separation step works more or less efficiently. Relatively low aluminum contentin combination with high polymer content can prevent aluminum from being separated [24].

Dimensionally stable plastics can be sorted by an NIR separator. Stable multilayer packagingshould be sorted into the rigid mixed plastics or remain in the residue.

Following sorting, the presorted polymers have to be recycled. This reprocessing of the sortedpolymers can be done in three ways:

1. energetic utilization is the recovery process of energy contained in the polymers by converting itinto thermal or electrical energy by incineration. Since this type of recycling does not deliver amaterial product, it will not be considered as recycling in the following text [26];

2. mechanical recycling, which describes the production of new products while the polymer chainsstay preserved. This is done by processing the polymer waste with physical methods likeshredding, solving, or melting [26,27].

3. chemical recycling, whereby polymer waste is turned back into its oil/hydrocarbon component inthe cases of polyolefins and into monomers in the case of polyesters and polyamides. These canbe used as raw materials for the production of new polymers [26].

The fractions containing multilayer packaging are partially incinerated and partiallyrecycled mechanically.

The presorted aluminum fraction, in which aluminum-containing multilayer packaging can alsobe contained, usually undergoes a pyrolysis step that removes the plastic components as well as thelacquers or residual contents, before the aluminum is remelted. In the case of a packaging in which thealuminum foil constitutes 19% and the plastic components make up the remaining 81%, the overallrecycling rate is 17%, whereby the loss of aluminum can be attributed to the formation of aluminumoxide [24].

The presorted LPB fraction is shredded and fed into a special paper treatment in which the fibersare dissolved and separated from the plastics and the aluminum, to be added to the fiber preparation.In some cases, the LDPE–Al reject is separated by selective dissolution, and the output can be utilizedto replace primary raw materials in high-quality applications. In most cases though, the utilization ofthe reject takes place in the rotary kilns of cement plants, where the plastic functions as fuel, and thealuminum replaces the aluminum ore Bauxite [24].

The sorting residue is incinerated completely, while mixed plastic fractions can either be recoveredby material recycling or by incineration, depending on the quality and on economic issues.

In the cases of the mechanical recycling of fractions in which multilayers are contained,the presorted material has to undergo a second sorting step. Here, material that is not polyolefin-basedhas to be removed. Therefore, the material is first shredded and washed, and afterwards it is treatedby sink-float separation. Since polyolefins are usually the target polymer, only the componentswith a density lower than 1 are processed in the following extrusion, while the components with adensity higher than 1 are incinerated. If a multilayered packaging has a high polyolefin content,it is possible that its density is low enough for the multilayer to enter the polyolefin recyclingflow. The non-polyolefinic material contained in those multilayers can be largely removed by meltfiltration [24].

In 2015, only 1.74% of the 2870 kt packaging waste produced in Germany was recovered bychemical recycling, which shows that this kind of waste recovery only plays a subordinate role in thecurrent market. Mechanical recycling (39.4%) and energy recovery (58.8%), however, are at competitivelevels (cf. Table 3) [11].

In the waste hierarchy (a set of priorities for the efficient use of resources) mechanical recycling isnumber three, after waste avoidance and reuse. However, mechanical recycling should be favoredover energy recovery and disposal. This hierarchy is quantified in several studies, as reported below.

Recycling 2018, 3, 1 8 of 26

Table 3. Treatment of packaging waste in Germany, 2015 [11].

Recovery Removal in kt/%

Total Mechanical Chemical Energetic Total Disposal Incineration Without Energy Recovery

2867 kt 1130 kt 50 kt 1687 kt 3 kt 3 kt 0 kt100% 39.4% 1.7% 58.8% 0.1% 0.1% 0.0%

One of the key benefits of recycling plastics is the reduction of the requirement of plasticsproduction [28]. Incineration has the potential to recover the chemical energy bound in the polymer,corresponding to the caloric value. Since most polymers have a caloric value in the same range asthat of crude oil (~40 MJ/kg) [29], plastic waste can be regarded as a crude oil substituent. However,it should be noted that the processing energy that is required to produce plastic items cannot berecovered by incineration. Mechanical recycling has the potential to save this processing energy aswell as the chemical energy. Depending on the type of polymer, the processing energy of polymersranges from 27 MJ/kg (PE) to 53 MJ/kg (PET) [29]. The ecologic benefit of mechanical recycling resultsfrom the savings of processing energy minus the energy used to collect, transport, and reprocess theplastic (~9 MJ/kg) [28,30–32].

Additionally, it must be taken into account that efficiencies of about 40% are considered to be theindustry standard of energetic recovery. While a prospective increase of the efficiency of incinerationplants would reduce the energetic advantage of mechanical recycling, the growing proportion ofrenewable energies in the energy mix, reduces the ecological benefit of the energetical utilization ofplastic waste [31,33–35].

In Germany, a new packaging law will enter into force in 2019, which sets new quotas concerningthe recycling of plastic packaging. The amount of recycled postconsumer plastic packaging waste hasto be increased from ~40% to 63% by 2022 [36]. In order to achieve this target, innovations concerningnew recycling and sorting technologies, as well as new strategies concerning packaging developmentare necessary.

Since multilayer packaging makes up a large percentage of the packaging market and is currentlymostly treated by incineration, it is an option to try reaching the quotas by pursuing a solution forthe recyclability dilemma of multilayer packaging. As described in Section 2.1.4, the replacementof multilayer packaging by monomaterials can be an option for some multilayer systems. However,it is expected that not all kinds of multilayer packaging can be substituted by monomaterials,and functionality, costs, and marketing are still the main drivers of the packaging market. Thus,it may be worthwhile having a closer look at the recycling strategies of multilayered packaging.

3. Recycling Methods

3.1. Theoretical Background of Polymer Blends and Solutions

In general, it can be said that there are two ways to recycle multilayered packaging items:the first option is to separate the different components and make them available for recycling inseparated recycling streams, the second option is to process the used components together in onecompatibilization step. For the separation of the different components, two methods are applied:a separation can be performed either by delamination of the system or by selective dissolution–precipitation of the different components.

While the delamination methods can be based on the chemical decomposition of an inter- oradhesive layer, methods based on selective dissolution and methods based on the combined processingof the different constituents can be described by the thermodynamics of polymer solutions.

Recycling 2018, 3, 1 9 of 26

In order to enable the spontaneous solubility of polymers in solvents or other polymers, the Gibbsfree energy of mixing ∆Gmix has to be zero or negative. ∆Gmix for the solution process is given by theGibbs–Helmholtz equation (Equation (1)) [37,38]:

∆Gmix = ∆Hmix − T·∆Smix, (1)

Since the gain in entropy ∆Smix is negligible because of the high molecular weight of the polymerchains, the enthalpy difference ∆Hmix of the blending process has to be negative, which means that theprocess has to be exothermic (c.f. Equation (1)) [37–39].

Flory and Huggins described ∆Gmix by the determination of the entropy of the mixing of anassembly of polymer chains and solvent. This is done by counting the number of ways solventmolecules and polymer segments, or two different polymer segments can be accommodated on alattice. According to the Flory–Huggins theory, ∆Gmix is generally defined as [40]:

∆Gmix = RT[

φ1

N1lnφ1 +

φ2

N2lnφ2 + φ1φ2χ12

], (2)

where Ni stands for the degree of polymerization of the polymer (i is the number of monomers perchain), φ1,2 is the volume fraction of the molecules, χ12 is the Flory–Huggins interaction parameter,R is the gas constant, and T is the absolute temperature.

The first two logarithmic terms of the Flory–Huggins-Equation give the combinatorial entropy ofmixing, which, by definition of Φ, is negative and always promotes mixing, while the third term is theenthalpy of mixing [40,41].

For polymer blends, Ni is large in both cases and, so, the combinatorial entropy becomesvanishingly small. Therefore, the miscibility behavior of the system is determined by the valueof the last term, i.e., the enthalpy of mixing. An exothermic heat of mixing may be caused by specificinteractions between the components of the blend, like covalent and ionic bonds. Weak, nonbondinginteractions such as hydrogen bonding, ion–dipole-, dipole–dipole- or donor–acceptor interactions orVan der Waals interactions often do not lead to polymer miscibility. This is why polymer miscibility ismore an exception rather than the rule [30].

In the case of N1 = 1, the general equation transforms into the equation for polymer solutions:

∆Gmix = RT[

φ1lnφ1 +φ2

N2lnφ2 + φ1φ2χ12

], (3)

Additionally, for polymer solutions, the combinatorial entropy promotes the solubility of thesystem. In the case of a solvent–solvent mixture, all three terms of the Flory–Huggins equationcontribute to solubility. This explains why many miscible solvents systems exist and why the solubilityof polymers, in general, is lower than the solubility of molecules with a low molecular weight [42].

As described in the following subsections, the formation of polymer solutions in both solvents andother polymers plays an important role for the recycling of multilayered polymer-based packaging.

3.2. Separation of Multilayer Components

This section is dedicated to methods that enable the separation of the different components ina multilayer packaging. A distinction is made between methods in which the target polymers arerecycled by dissolution and precipitation, and those that perform a separation of the different layersby delamination.

3.2.1. Recycling of a Target Polymer by Selective Dissolution–Reprecipitation

Pioneer work concerning the recycling of polymer mixtures by selective dissolution was doneby Nauman et al. [43] in 1992. Here, the different polymers of the mixture are dissolved at a certaintemperature one after another, eventually even in the same solvent. The solution of the first polymer is

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separated from the residue by filtration and eventually additivated, before the polymer is recovered byconventional techniques for extracting a polymer from a solvent. The solvents used in the study were,among others, tetrahydrofuran, xylene, or toluene. This scheme was also explicitly tested for multilayerstructures. As expected, the selective dissolution process worked for bilayer materials, since bothpolymers were in contact with the solvent. Surprisingly, it also worked on multicomponent materialswhere the inner polymer was well shielded from the solvent by an outer polymer with a solutiontemperature higher than that of the inner polymer. Instead of codissolving at the higher temperature,the inner layers were not well shielded from the solvent, and selective dissolution could occur evenwhen the ordering of the individual polymers was incorrect based on the criterion that the outermostlayers must dissolve at the lowest temperatures. It appears that the diffusion of the solvent through theundissolved outer layers causes delamination at the internal polymer-to-polymer interfaces, exposingthe entire structure to the solvent. Delamination has also been observed at polymer–metal interfaces.The multilayer structures tested where PP/Maleated-PP, Tie Layer/EVOH/Maleated-PP, Tie Layer/PP,and PA-6/Tie/EVOH/EVA/Surlyn® [43].

In 1995, a patent specification was published that reports on the extraction of polyolefins fromcomposite packaging containing one or several synthetic polymers as well as metals or naturalpolymers. As solvent cycloalkanes, n-alkanes or iso-alkanes, and mixtures thereof can be used.After removal of the nonsoluble components, the solution is dispersed in an aqueous tenside solution.In this way, a dispersion consisting of the precipitated polymer, an aqueous phase, and an organicphase are produced. The precipitation of the polyolefin is nearly quantitative, and the size of thepolyolefin particles can vary between 0.01 and 8.00 mm, with the particles having a very smooth surface.A further advantage is that the number of washing steps and also the costs and time expenditure canbe reduced by this precipitation method [44].

In 2001, a method was introduced by Mäurer et al. [45] for the treatment of polymer-containingwaste, which was also specified for packaging recycling [46]. In the case of packaging recycling,the method aims to recover PE, the polymer component that has the highest value-adding potential,because of its big mass fraction. PE can be recovered in a quality close to that of the virgin material,while the undissolved components remain as a residue of little value. Unlike Nauman’s and otherearlier patents, solvents applied by Mäurer et al. are not classified as hazardous materials and thereforedo not require special labelling. Additionally, a better recovery of the solvent is achieved. In 2017,it was announced that the method was going to be realized in pilot scale in Indonesia to recycle plasticsachet waste from landfills [47]. Aside from packaging recycling, this method is also applied to thewaste of electrical and electronic equipment, end-of-life vehicles, and construction materials [48–51].

In the literature, several more patents describing the dissolution–reprecipitation method forcommingled postconsumer packaging waste can be found, even though multilayered packaging is notmentioned directly [52–54].

Lindner et al. [52] described a method to obtain LDPE from presorted plastic films, consistingof four steps for the removal of inks, removal of low-molecular-weight components, solution of thefilm components, precipitation and removal of undesirable polymers, and finally recovering of LDPEfrom the solution [52]. In WO 2000077082 A1 [53], a patent specification from 2000, the polyolefiniccomponents were dissolved from commingled postconsumer plastic packaging and separated fromthe undissolved residue by common mechanical separation technologies. Afterwards, the differentpolyolefins HDPE, PP, and LDPE were successively precipitated from the solution by crystallization,by applying shear forces at different temperatures, while oligomers, inks, and additives were intendedto remain in the solution [53]. WO2005118691A2 [54], a patent specification from 2006, described therecycling of PA-6 and PA-6,6 from commingled polyolefin polymer waste. By admixing the waste withan ester solvent and heating the admixture to a temperature above the melting temperature of thecontained polymer, an ester solvent composition with dissolved polyamide and a separate immiscibleliquid polyolefin phase was formed. The separation of the discrete molten polyolefin phase from the

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ester solvent composition could be performed by skimming, decantation, filtration, centrifugation,or combinations thereof [54].

Additionally, different descriptions of the recovery of packaging-relevant polymers from differentpolymer mixtures, not explicitly originating from packaging waste, exist [55–57].

One advantage of the dissolution–precipitation method is that the input does not have to runthrough a complex sorting scheme. The input, thus, can consist of a heterogeneous mixture of differentmultilayer and non-multilayer packaging systems. Additionally, the precipitated polymer can beexpected to be of very high-quality, even competing with the virgin polymer. The biggest drawbacks ofthis method are the energy-intensive drying of the polymer and the fact that all polymer componentsthat do not dissolve remain as a residue of little value.

3.2.2. Delamination of Multilayer Packaging

The mechanism of multilayer delamination can be induced physically by the dissolution ofmacromolecules, and mechanically or chemically by the decomposition of an interlayer or by reactionsat the interface.

In the literature, several methods of physical delamination by dissolution of interlayers arepresented. The TetraPak patent of 1996 described a multilayer packing comprising PVOH or otherwater-soluble, thermoplastic resin layers disposed on both sides of a paperboard base material layer,and other thermoplastic synthetic resin layers laminated thereon. An aluminum foil layer may alsobe contained over a polyolefin adhesive resin layer. The advantage of this composition is that, in hotwater, the polyvinyl alcohol or another water-soluble, thermoplastic resin promptly dissolve on bothsides of the paperboard base material layer. Thus, a fast separation of the components and a low fibercontent in the resins can be achieved [58].

A similar method was described by DE 4328016 A1 [59], a patent specification from 1994,that described a multilayer structure which contains at least three layers, whereof two layers (that areinsoluble in a non-neutral aqueous medium) have to be separated by one layer of a polymer that issoluble in non-neutral aqueous solution. This separation layer can consist of at least one acrylate,one aminoacrylate, and, optionally, one neutral vinyl monomer [60]. Such polymers are insoluble inbasic solutions, but soluble in acidic solutions. An example of a polymer suitable to be a separationlayer is a copolymer of ethyl acrylate and methacrylic acid [61]. To facilitate the separation of thedelaminated components of a multilayer with, for instance, five layers, different separation layers canbe installed [62].

In 2004, Mukhopadhyay [63] introduced a method in which the components of an aluminum–plastic packaging could be recovered by delamination induced by 50–70% nitric acid. In this medium,neither aluminum nor the plastic component were affected, but the binder adhesive was dissolved.The delaminated constituents were separated by their specific gravity in a series of baths. The timeneeded for the delamination process depends on the size of the film fragments and the concentration ofnitric acid. Stripes of a width of 0.25 cm and a length of preferably less than one meter take about fourto seven hours to completely delaminate the structure. In contrast to hydrochloric acid and sulfuricacid, nitric acid does not dissolve aluminum, because a passivating Al2O3-layer is formed [63].

Alkali-soluble polymers can also be used for water-resistant labels that can easily be removed inalkaline milieus [61].

A patent specification of Kernbaum and Seibt [64] from 2013 described another method for theremoval of the bonding agent between the different layers. As separating fluids, nanoscaled dispersionsor precursors of these nanoscaled dispersions were used that contained an organic solvent, a waterycomponent, and at least one stabilizing amphiphilic surfactant. With the help of this separating fluid,the separation of adhesions and coatings is possible by reducing interfacial tensions between thephases of glued and coated materials, thus causing separation. The solid, delaminated components ofthe multilayer system are usually separated by a swim-sink method, but also by using the differencesof the magnetic or electrical properties of the materials [64].

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Delamination by mechanical impact is an exception, since adhesion between the different layersin a multilayer packaging usually is too strong.

In 2016 Perick et al. [65], however, published a packaging that comprised a transparent outerlayer and an inner layer that usually consists of a material different from the outer layer. A specialcharacteristic of the packaging is that the inner and outer layer were not connected together for atleast 30% and preferentially even over 70%. Shredding of the multilayer provided separated particles,at least at the areas where both layers were not connected by an adhesive. Those single-materialparticles can be separated by common techniques. The printed layer is exposed after the shredding ofthe multilayer and can be washed off with water [65].

In the following, an overview of the recycling methods is given in which the delamination, andthus the separation of the components, is obtained by the chemical or biochemical degradation of one(inter)layer of the multilayer or by reactions at the interface.

A method introduced by Patel et al. [66] in 2016 addressed a multilayer packaging consisting ofPET and PE. Here, sulphuric acid with concentrations ranging from 68 to 98% was used to degradethe PET component. The PE film was not affected and could be reused after several washing steps.Since no information was given on the utilization of the decomposition products of PET, probably nonoteworthy reuse is possible [66].

In 2011, Kulkarni et al. [67] proposed the recovery of aluminum from multilayeredplastic–aluminum packaging structures by decomposition of the polymeric components in a sub-and supercritical water process. In comparison to polyolefins, condensation polymerization polymerssuch as PET or PA can be depolymerized into their monomers comparatively easy at subcriticalconditions. This method leaves pure aluminum [67].

Another method to chemically delaminate multilayer packaging structures is by enzymaticdecomposition of a bio-based interlayer. Multilayer films were produced that were composed of PEand PET films with an interim barrier layer based on a whey protein isolate that also achieves valuablebarrier properties suitable for modified atmosphere packaging [68,69]. Whey proteins can be degradedby enzymatic hydrolysis. Because of this biochemical treatment, the coating layer can be washed offfrom the plastic substrate layer. The washing of samples based on PET–whey and PET–whey–PE wasefficient when performed with an enzymatic detergent containing protease enzymes. Different types ofcommercial enzymatic detergents presented positive results in removing the protein layer from the PETsubstrate and from the sandwich films, achieving detachment from the PET and PE films. This allowedfor further separation of the different polymer films by density separation. The mechanical propertiesof the plastic substrate, such as stress at yield and stress and elongation at break, were evaluated bytensile testing on films before and after cleaning. Hereby, no significant affection from washing withenzymatic detergents could be observed [68,70].

Additionally, some delamination strategies based on the decomposition of an aluminum layer innon-neutral watery solutions are described in the literature.

Le et al. [71] and Benzing et al. [72] both offered an option to recycle multilayer packagingcontaining aluminum films. One aspect of the method was the separation of the components byselective dissolution of the intermediate aluminum layer. Aluminum was reacted with an alkali oracid aqueous solution to be dissolved in an aqueous solution phase. Generally, the reaction rate ofaluminum with an alkali is higher than that of aluminum with an acid. The reaction rate depends onthe concentration and temperature of the alkali or acid aqueous solution and on the rate of diffusioncaused by agitation. A reaction equation of the alkali aqueous solution, which is exemplified bysodium hydroxide (NaOH) aqueous solution and aluminum, is as follows [71,72].

2 Al + 2 NaOH + 6 H2O→ 2 NaAl(OH)4 + 3 H2

In other words, aluminum is dissolved in a sodium aluminate state in the aqueous solution,and hydrogen gas is generated as a result of the reaction. The soluble sodium aluminate can be

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precipitated as poorly soluble aluminum hydroxide. According to the Bayer process, aluminumhydroxide can be converted to Al2O3 and subsequently reduced to elemental aluminum.

According to Le’s [71] invention, the other layers include PET, PP, and PE. The polyolefinlayers can be separated from the PET film by using different gravities, and in order to ensure acomplete removal of PET and PO from the respective other fraction, an extraction step can be installeddownstream. According to Benzing et al. [72], a paperboard layer can also be contained in themultilayer. Mukhopadhyay et al. [73], however, also described a recovering method of aluminum,paperboard, and polymer-containing multilayer structures by a basic solution. Again, the aluminumwas recovered as a water-soluble salt, while the cardboard and plastic component could be separatedby their differences in density [73].

Another method to induce delamination of multilayer packaging, can be by a chemical reaction atthe interface between two layers. Several publications describe the separation of polymer–aluminummultilayer packaging by the use of acids. Polymer–aluminum multilayers, for example, are used in thecase of LPBs. Here, at first, the cardboard component has to be removed in a watery medium.

Fowkes et al. [74,75] described the delamination mechanism of a PE–Al film through fatty acidsin 1982. In this case, acid and water molecules diffused through the LDPE layer to the interface.Adhesion at the interface consisted mainly of ionic bonds, hydrogen bonds, and Van der Waals forcesbetween aluminum oxide and oxygen-containing groups of the adhesive polymer like alcohols, ketones,aldehydes, or carboxylic acid on the surface of the polymer [74–76]. Thus, the adhesion between the Alfoil and LDPE was mainly due to ionic and hydrogen bonds and Van der Waals forces between theiroxidized species. Since the aluminum oxide layer can be dissolved at pH = 4, elementary aluminumis laid open and is able to react with the aqueous acid under formation of hydrogen and aluminumacetate [77].

Al(s) + 3H3O+(aq) →

32

H2(g) + Al3+(aq) + 3H2O

Al3+ + 3CH3COO−(aq) → Al(OOCCH3)3(s)

The sorption of the acids to the polymer and aluminum surface can induce delamination byreducing interlayer adhesion.

The sorption and the solubility in the polymer were affected by the degree of saturation of thefatty acids. Since the sorption increases with the amount of double bonds but the rate of diffusiondecreases with increasing numbers of double bonds because of the increasing rigidness, monosaturedor nonsaturated acids are to be preferred to induce delamination [77]. Nonetheless, this diffusivedelamination mechanism requires long reaction times and has a low separation efficiency, which iswhy further measures have to be taken for recycling applications [76].

Early patents already described the separation of the different layers of laminated packaging withacidic solvent mixtures [78,79]. The waste was treated with an organic acid or a mixture of organicacids e.g., acetic acid. In 1995 Johansson et al. [78] introduced a method that was carried out at a hightemperature (80 ◦C), close to the flash point of acetic acid, which not only required a large amountof energy, but also added a safety risk. The mixture used is highly aggressive because of the highconcentration (80%) of acetic acid. This mixture will attack the aluminum components and lead to theformation of hydrogen, as well as to a loss of the amount of aluminum recovered in the process [78].

Kersting et al. [79] published a method for separating an aluminum foil from plastic films, such asPE films, in 1992. The laminates were placed in a 20% solution of low fatty acids (e.g., acetic acid,propionic acid, formic acid, butonoic acid) and heated to 100 ◦C for 10–20 min. An important aspect ofthe invention is also that the cooling process was carried out in a closed vessel, whereby a reducedpressure of about 200 hPa could form. Air and solvent traces between the plastic and the aluminum foilexpanded because of the decrease of pressure and thus promoted a separation of the components [79].

In a more recent patent from 2012 of Bing et al. [80], a similar delamination method utilizingorganic acids (e.g., acetic or formic acid) was described. As a difference, an additional organic solvent(e.g., acetone, chloroform) that works as a swelling agent to accelerate diffusion, and a surfactant

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(e.g., polyoxyethylene sorbitan monolaurate) to decrease the surface tension and thus increase thewettability, were used here. Plastic–aluminum composites like toothpaste housings, can be separatedby using a mixed separating agent consisting of 40–70% organic acid, 10–20% surfactant, and 20–40%organic solvent [80].

In 2015, Lovis et al. [81] introduced a method that is based on the usage of a microemulsioncomprising swelling agents, carboxylic acids, water, and surfactants. These microemulsions hadinterfacial tensions low enough to penetrate the interphase and counteract adhesion [81].

Delamination of aluminum-containing packaging was also reported in a case of damage causedby the ingredients of cosmetic products. Here, compounds comprising a phenyl and a hydroxyl groupcaused delamination by diffusion to the aluminum–polymer interface [82,83].

The methods that provide the delaminated multilayer components have in common that differentmaterials have to be separated from each other to be recycled in single-material streams. This caneasily be performed by electrostatic separation if one component is aluminum and the second one is apolymer. The separation of the different components proves to be rather difficult if a polymer/polymermixture is present. For the conventional NIR-separation technique, a ballistic trajectory has to becalculated. This is difficult for the delaminated components, since delaminated layers can be verythin (layers in multilayer packaging often have a thickness of 5 to 30 µm) and often are cut into smallpieces to make intermediate layers and interfaces accessible for delamination treatment. Separation bydensity also proves to be difficult. Using the float–sink technique, pieces may be undesirably separated.For example, if air bubbles are attached to a surface of the PET layer with a density of 1.38 g/cm3, or ifthe PET layer is positioned on a plurality of PP or PE (δ = 0.9–0.97 g/cm3) films, it will float on water.If a polyolefinic layer is positioned under a plurality of PET layers, it will sink in water. These mistakescan be reduced through the addition of a surfactant or through the repetition of the separation processusing the specific gravity differences.

If incorrectly incorporated material has a melting point higher than the melting point of the actualmaterial flow, it can be caught in a melt filtration step by a screen or mesh provided on a breakerplate between a screw and a die. The entry of the component with the highest melting point occurs independence upon the size of the mesh.

The share of low-density plastics contained in the sink fraction is usually constituted by polyolefinsif water is used as a separation medium. Since those have a melting point lower than PE and PA,they cannot be removed by melt filtration. Instead, the polyolefins can be dissolved selectively usingan organic solvent to extract them from the sink fraction. As an alternative to the removal of theimpurities, a compatibilization step is also conceivable.

The costly and time-consuming separation step is in competition with the quality of therecycled material.

For multilayers that are delaminated into more than two different polymer types, and mixtures ofmultilayers containing more than one type of multilayers, the purification becomes even more complex.Eventually, new flake-sorting technologies could also be applied here.

The scenario of commingled postconsumer multilayer packaging by delamination would requirea separate sorting path for multilayers and thus a marker that makes them sortable.

3.3. Combined Processing

The blending of polymers is an approach to recycling polymer-based multilayers withoutseparating the components. However, because of insufficient solubility, most blends have poormechanical properties and unstable morphologies, which is why the addition of additives is necessary(cf. 3.1).

In contrast to miscible polymers, immiscible blends do not involve thermodynamic solubilityand cannot be characterized by the presence of one phase and one single glass transition temperature.Their overall behavior depends upon the properties of the single components and their ratio, as well

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as the morphology of the blend and the properties of the interface. Those factors complicate theprediction of blend properties.

Compatibilization is a process and technique for improving a blend performance by makingblend components less immiscible through the addition or in situ generation of a macromolecularspecies that exhibits interfacial activity in heterogeneous polymer blends. Those compatibilized blendsconsist of one component finely dispersed in the other, with good adhesion of the phases and strongresistance to coalescence. Therefore, the material can be considered macroscopically homogeneous.

As compatibilizers, macromolecular chains with a blocky structure, such as block or graftcopolymers, are often used. One of the blocks is miscible with one blend component, and a secondblock is miscible with the other blend component. Polyolefins grafted or copolymerized with moleculescontaining anhydridic, acidic, or ionic groups are often used as well.

The theory and mechanism of compatibilizing is described in the literature in detail [37,84],and recycling-related compatibilization is discussed [38].

Ragaert et al. [10] provided a summary of typical compatibilizers used to process nonmisciblepolymers to produce a blended material. In the following, concrete examples of multilayer packagingrecycling are presented.

3.3.1. Recycling of Multilayers Containing PE/PA

As can be seen in Table 2, laminates containing PA and polyolefins play an important role.The blending of polyolefins with PA produces thermodynamically immiscible two-phased systems [85–87].In literature, several techniques for the compatibilization of those blends can be found.

A method commonly used to induce the compatibility between PA and polyolefins is by additionof the polyolefin component grafted with maleic anhydride (PE/PP-g-AH). This concept was employedby Choudhury et al. [88] in 2005 to recycle a postconsumer oil pouch material consisting of a coextrudedfilm made of LDPE, LLDPE, and PA-6. During the melt extrusion of the pouch components togetherwith the compatibilizer, intermolecular interactions between the primary amine end group of PA-6and the anhydride groups can take place. The polyethen backbone of the PE-g-AH copoylmer is ableto form a Van der Waals interaction with the polyolefinic contents of the pouch (cf. Figure 3). Duringthe formation of the imide, one equivalent of water is formed as a by-product. In order to shift theequilibrium to the side of the imide, the removal of water is necessary. This requires an extruder withventing capabilities.

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of a coextruded film made of LDPE, LLDPE, and PA-6. During the melt extrusion of the pouch

components together with the compatibilizer, intermolecular interactions between the primary amine

end group of PA-6 and the anhydride groups can take place. The polyethen backbone of the PE-g-

AH copoylmer is able to form a Van der Waals interaction with the polyolefinic contents of the pouch

(cf. Figure 3). During the formation of the imide, one equivalent of water is formed as a by-product.

In order to shift the equilibrium to the side of the imide, the removal of water is necessary. This

requires an extruder with venting capabilities.

Figure 3. Compatibilization mechanism of PE and PA using PE-g-MA. Adapted from [89].

With this method, tensile strength, hardness, and percent elongation-at-break of the

compatibilized blend could clearly be improved by the increased interfacial adhesion.

Dagli et al. [90] used the same technique and recycled PP/PA blends by using PP-g-AH

copolymer as a compatibilizer in 1994.

Another compatibilizer tested for the LLDPE/LDPE/PA oil pouch was an ionomer of an

ethylene–methacrylic acid copolymer. The intermolecular interactions expected to cause the decrease

of interfacial tension between the different phases in this case are depicted in Figure 4 [88].

Figure 4. Compatibilization mechanism of PE and PA using an ionomer. Adapted from [88].

Blends compatibilized by the ionomer exhibit a higher degree of crystallinity, which causes even

better tensile properties than the blends compatibilized by PE-g-AH.

Figure 3. Compatibilization mechanism of PE and PA using PE-g-MA. Adapted from [89].

With this method, tensile strength, hardness, and percent elongation-at-break of the compatibilizedblend could clearly be improved by the increased interfacial adhesion.

Dagli et al. [90] used the same technique and recycled PP/PA blends by using PP-g-AH copolymeras a compatibilizer in 1994.

Another compatibilizer tested for the LLDPE/LDPE/PA oil pouch was an ionomer of anethylene–methacrylic acid copolymer. The intermolecular interactions expected to cause the decreaseof interfacial tension between the different phases in this case are depicted in Figure 4 [88].

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Recycling 2017, 2, 25 15 of 25

of a coextruded film made of LDPE, LLDPE, and PA-6. During the melt extrusion of the pouch

components together with the compatibilizer, intermolecular interactions between the primary amine

end group of PA-6 and the anhydride groups can take place. The polyethen backbone of the PE-g-

AH copoylmer is able to form a Van der Waals interaction with the polyolefinic contents of the pouch

(cf. Figure 3). During the formation of the imide, one equivalent of water is formed as a by-product.

In order to shift the equilibrium to the side of the imide, the removal of water is necessary. This

requires an extruder with venting capabilities.

Figure 3. Compatibilization mechanism of PE and PA using PE-g-MA. Adapted from [89].

With this method, tensile strength, hardness, and percent elongation-at-break of the

compatibilized blend could clearly be improved by the increased interfacial adhesion.

Dagli et al. [90] used the same technique and recycled PP/PA blends by using PP-g-AH

copolymer as a compatibilizer in 1994.

Another compatibilizer tested for the LLDPE/LDPE/PA oil pouch was an ionomer of an

ethylene–methacrylic acid copolymer. The intermolecular interactions expected to cause the decrease

of interfacial tension between the different phases in this case are depicted in Figure 4 [88].

Figure 4. Compatibilization mechanism of PE and PA using an ionomer. Adapted from [88].

Blends compatibilized by the ionomer exhibit a higher degree of crystallinity, which causes even

better tensile properties than the blends compatibilized by PE-g-AH.

Figure 4. Compatibilization mechanism of PE and PA using an ionomer. Adapted from [88].

Blends compatibilized by the ionomer exhibit a higher degree of crystallinity, which causes evenbetter tensile properties than the blends compatibilized by PE-g-AH.

Additionally, several patents describing the recycling of multilayered films based on polyolefinsand polyamide can be found in the literature. In DE 3938552, a patent specification from 1991compatibilization was induced by radical cross-linking between the different polymer typesby using peroxides and coagents like 2-butyne-1,4-diol. The diol reacts with the secondaryamide of the polyamide, and the triple bond promotes radical cross-linking with the polyolefin.The polyethylene/polyamide blend was diluted with polypropene to adjust the mechanical propertiesand the melt viscosity [91]. In DE 4223864 A1, multilayers consisting of PA and PE (0.5–50%) weremixed to PA (40–98%) together with 1–50% compatibilizer. An additional 0–60% could be fillermaterials. The compatibilizers used were polyethenes of different molecular weights, which werefitted either by the grafting of PE or by the copolymerization of ethene with other monomers reactivewith respect to PA (anhydrides, amino, acid, epoxide, ester groups, and salts of carboxylic acids).The products obtained could be used as a replacement material for PA [92].

In EP 0575855A1 [93], a patent specification published in 1993, washed and unsorted plastichousehold waste, and the swimming fraction of the plastic waste treatment were treated. The processingwas performed in a polymer melt in the presence of unsaturated monomers and radical initiators.Initiated by the peroxide, the monomers grafted onto the different polymer components. Thus,different graft copolymers with different graft matrices but the same sidechains resulted. Hence,the incompatibility of the different polymers could be reduced and an improvement of strength andtoughness could be achieved [93].

3.3.2. Recycling of Multilayers Containing PE/PET

Like PA, PET is often used in packaging applications in combination with polyolefins but doesnot exhibit miscibility. Again, polyolefins grafted with maleic anhydride (PE/PP-g-MA) can be usedto compatibilize PET and PO. The anhydride groups of the compatibilizer can react with the PET

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hydroxyl end group in an esterification, promoting a chemical anchoring between the polyester and thecompatibilizer (cf. Figure 5). The polyolefin phase of PE/PP-g-MA is miscible with the PO component,allowing for physical anchoring [94].

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Additionally, several patents describing the recycling of multilayered films based on polyolefins

and polyamide can be found in the literature. In DE 3938552, a patent specification from 1991

compatibilization was induced by radical cross-linking between the different polymer types by using

peroxides and coagents like 2-butyne-1,4-diol. The diol reacts with the secondary amide of the

polyamide, and the triple bond promotes radical cross-linking with the polyolefin. The

polyethylene/polyamide blend was diluted with polypropene to adjust the mechanical properties

and the melt viscosity [91]. In DE 4223864 A1, multilayers consisting of PA and PE (0.5–50%) were

mixed to PA (40–98%) together with 1–50% compatibilizer. An additional 0–60% could be filler

materials. The compatibilizers used were polyethenes of different molecular weights, which were

fitted either by the grafting of PE or by the copolymerization of ethene with other monomers reactive

with respect to PA (anhydrides, amino, acid, epoxide, ester groups, and salts of carboxylic acids). The

products obtained could be used as a replacement material for PA [92].

In EP 0575855A1 [93], a patent specification published in 1993, washed and unsorted plastic

household waste, and the swimming fraction of the plastic waste treatment were treated. The

processing was performed in a polymer melt in the presence of unsaturated monomers and radical

initiators. Initiated by the peroxide, the monomers grafted onto the different polymer components.

Thus, different graft copolymers with different graft matrices but the same sidechains resulted.

Hence, the incompatibility of the different polymers could be reduced and an improvement of

strength and toughness could be achieved [93].

3.3.2. Recycling of Multilayers Containing PE/PET

Like PA, PET is often used in packaging applications in combination with polyolefins but does

not exhibit miscibility. Again, polyolefins grafted with maleic anhydride (PE/PP-g-MA) can be used

to compatibilize PET and PO. The anhydride groups of the compatibilizer can react with the PET

hydroxyl end group in an esterification, promoting a chemical anchoring between the polyester and

the compatibilizer (cf. Figure 5). The polyolefin phase of PE/PP-g-MA is miscible with the PO

component, allowing for physical anchoring [94].

Figure 5. Chemical reaction between the terminal hydroxyl groups of PET with maleic anhydride

groups of PE-g-MA [95].

In 2001, Uehara et al. [94] recycled postindustrial plastic films containing 70% of PE, 10% of PET,

10% of PA-6, and 10% adhesives and ink by using PE-g-MA. Because the reaction of the succinic ring

of the maleic anhydride with the terminal secondary amine PA is very fast [96,97], PE-g-MA first

compatibilizes the PA component and, subsequently, the PET component. This causes poor

mechanical properties at low PE-g-MA contents (3–5%), where the PET component stays

uncompatibilized, and a sudden increase to 5 to 10%.

Additionally, the compatibilization effect of a copolymer of ethylene–glycidyl methacrylate (E-

GMA) was investigated. In the case of E-GMA, the preferred reaction of the glycidyl groups involves

the COOH end group of PET [98]. By increasing the concentration of E-GMA, however, the

competition among the reactions between PA and both PET terminal groups can be overcome. Even

though an increase of the concentration of E-GMA to 15% overcomes this competition between the

reactions, the mechanical properties of the material compatibilized by 15% E-GMA are not

competitive with the properties of the material compatibilized with 15% PE-g-MA. This is due to a

cross-linking effect of E-GMA that has to be taken into account at higher E-GMA concentrations. The

Figure 5. Chemical reaction between the terminal hydroxyl groups of PET with maleic anhydridegroups of PE-g-MA [95].

In 2001, Uehara et al. [94] recycled postindustrial plastic films containing 70% of PE, 10% of PET,10% of PA-6, and 10% adhesives and ink by using PE-g-MA. Because the reaction of the succinic ringof the maleic anhydride with the terminal secondary amine PA is very fast [96,97], PE-g-MA firstcompatibilizes the PA component and, subsequently, the PET component. This causes poor mechanicalproperties at low PE-g-MA contents (3–5%), where the PET component stays uncompatibilized, and asudden increase to 5 to 10%.

Additionally, the compatibilization effect of a copolymer of ethylene–glycidyl methacrylate(E-GMA) was investigated. In the case of E-GMA, the preferred reaction of the glycidyl groupsinvolves the COOH end group of PET [98]. By increasing the concentration of E-GMA, however,the competition among the reactions between PA and both PET terminal groups can be overcome.Even though an increase of the concentration of E-GMA to 15% overcomes this competition between thereactions, the mechanical properties of the material compatibilized by 15% E-GMA are not competitivewith the properties of the material compatibilized with 15% PE-g-MA. This is due to a cross-linkingeffect of E-GMA that has to be taken into account at higher E-GMA concentrations. The reactionsdepicted in Figure 6 give rise to a complex macromolecular structure that has a direct impact on theviscosity and the mechanical properties of the blend [94].

Recycling 2017, 2, 25 17 of 25

reactions depicted in Figure 6 give rise to a complex macromolecular structure that has a direct impact

on the viscosity and the mechanical properties of the blend [94].

Figure 6. Cross-linking mechanism of E-GMA with PET. The hydroxyl groups marked in red show

possible reaction sites for further cross-linking reactions. Adapted from [94].

In contrast to E-GMA, PE-g-MA cannot induce cross-linking, since it can only react with the

terminal hydroxyl group of PET. Thus, for the recycling of scraps containing polyolefins and PET,

the usage of PO-g-MA seems to be a good method, especially if higher amounts of compatibilizer are

needed [94].

In 2000, Wyser et al. [100] investigated the recycling of a multilayer packaging material

consisting of PP–PET–SiOx by using a maleic acid-grafted polyolefin component, in which the

polyolefin component was PP. The compatibilization mechanism is already depicted in Figure 5. While

the unmodified recycled PP–PET–SiOx blend exhibited a coarse morphology and was brittle because

of a lack of adhesion between the two polymer phases, the compatibilized material showed a fine

blend morphology, with PET domains smaller than 7 mm at concentrations of 5 and 10%. Since,

however, at concentrations above 5% of PP-g-MAH, a brittle interphase started forming between the

PP and the PET, with a corresponding drop in the ductility, the best results with a fine morphology

and excellent ductility were obtained with a high content of PP-g-MAH [99].

According to this study, the size of the SiOx inclusions, resulting from the fragmentation of the

oxide coating during reprocessing, did not influence the mechanical properties of the blend, as long

as the concentration was below 2 × 10−3 [99,100].

Like for polyamides, ionomers can also be used for compatibilization. Choudhury et al. [100]

investigated the recyclability of LDPE/LLDPE/PET-based postconsumer laminated oil pouches by

the addition of a zinc salt of ethylene methacrylic acid copolymer (Surlyn® ionomer) in comparison

to PE-g-MAH (Fusabond® ). Again, because of its higher crystallinity, the blend compatibilized by the

ionomer exhibited better tensile properties compared to blends implemented by the anhydride

grafted additive. Also, the increase of the impact strength and hardness was more effective with the

Surlyn® ionomer than with Fusabond® for the system studied [100].

The technique of compatibilization is uncomplicated in application, since it can be performed in

a melt-processing step like extrusion. One disadvantage of this method is that, in order to add the

optimal amount of compatibilizer, the composition of the input has to be known. While the addition

of too little compatibilizer provides unsatisfactory product properties, the addition of an excess

amount results in unnecessary costs due to high-priced compatibilizers. In order to get a consistent

product quality that is usually desired by the purchaser, the composition of the input material has to

be consistent. Thus, in multilayer recycling, this method, above all, is suitable for postindustrial

waste, like edge trim and start up waste, that accounts for about 10–15% of the input. Here, the

proportion of the components is known and constant in composition. Additionally, compatibilization

only adds one additional life cycle to the polymer and thus it does not promote a circular economy

Figure 6. Cross-linking mechanism of E-GMA with PET. The hydroxyl groups marked in red showpossible reaction sites for further cross-linking reactions. Adapted from [94].

In contrast to E-GMA, PE-g-MA cannot induce cross-linking, since it can only react with theterminal hydroxyl group of PET. Thus, for the recycling of scraps containing polyolefins and PET,

Recycling 2018, 3, 1 18 of 26

the usage of PO-g-MA seems to be a good method, especially if higher amounts of compatibilizer areneeded [94].

In 2000, Wyser et al. [99] investigated the recycling of a multilayer packaging material consistingof PP–PET–SiOx by using a maleic acid-grafted polyolefin component, in which the polyolefincomponent was PP. The compatibilization mechanism is already depicted in Figure 5. While theunmodified recycled PP–PET–SiOx blend exhibited a coarse morphology and was brittle because of alack of adhesion between the two polymer phases, the compatibilized material showed a fine blendmorphology, with PET domains smaller than 7 mm at concentrations of 5 and 10%. Since, however,at concentrations above 5% of PP-g-MAH, a brittle interphase started forming between the PP and thePET, with a corresponding drop in the ductility, the best results with a fine morphology and excellentductility were obtained with a high content of PP-g-MAH [100].

According to this study, the size of the SiOx inclusions, resulting from the fragmentation of theoxide coating during reprocessing, did not influence the mechanical properties of the blend, as long asthe concentration was below 2 × 10−3 [99,100].

Like for polyamides, ionomers can also be used for compatibilization. Choudhury et al. [100]investigated the recyclability of LDPE/LLDPE/PET-based postconsumer laminated oil pouches by theaddition of a zinc salt of ethylene methacrylic acid copolymer (Surlyn® ionomer) in comparison toPE-g-MAH (Fusabond®). Again, because of its higher crystallinity, the blend compatibilized by theionomer exhibited better tensile properties compared to blends implemented by the anhydride graftedadditive. Also, the increase of the impact strength and hardness was more effective with the Surlyn®

ionomer than with Fusabond® for the system studied [100].The technique of compatibilization is uncomplicated in application, since it can be performed

in a melt-processing step like extrusion. One disadvantage of this method is that, in order to add theoptimal amount of compatibilizer, the composition of the input has to be known. While the addition oftoo little compatibilizer provides unsatisfactory product properties, the addition of an excess amountresults in unnecessary costs due to high-priced compatibilizers. In order to get a consistent productquality that is usually desired by the purchaser, the composition of the input material has to beconsistent. Thus, in multilayer recycling, this method, above all, is suitable for postindustrial waste,like edge trim and start up waste, that accounts for about 10–15% of the input. Here, the proportionof the components is known and constant in composition. Additionally, compatibilization only addsone additional life cycle to the polymer and thus it does not promote a circular economy in theproper meaning of the word. The end-of-life treatment of material recycled by compatibilization isincineration, since it cannot be assigned to a sorting stream.

The direct incorporation of compatibilizers into multilayer structures, however, allows for theutilization of this method for postconsumer waste. In 2016, Dow introduced a self-recyclable multilayerstructure packaging that already containes a compatibilizer and even claimed to show improvedcompatibilization, when compared to adding a compatibilizer component as a separate stream ina recycling process. In the multilayer structure, at least one polyolefin component, at least onelayer consisting of a polar polymer (e.g. PA, PET), and at least one tie layer consisting of a maleicanhydride-grafted polymer had to be present. The compatibilizer, which can be an anhydride and/orcarboxylic acid-functionalized ethylene–alpha-olefin copolymer, was present in the range of 1% to35% in the polyolefin layer,. Examples are Retain 3000, a maleic anhydride-grafted ethylene–octeneplastomer, or AmplifyTM TY, a MAH-grafted polymer [101].

3.3.3. Others

In the literature, several further descriptions of polymer packaging recycling containing severalpolymer types can be found that are not explicitly meant for multilayered packaging. Nonetheless,the strategies described are similar to the methods mentioned before, and mostly polyolefin-basedcompatibilizers grafted with anhydrides or acids can be found [102–105].

Recycling 2018, 3, 1 19 of 26

Additionally, other different descriptions of the blend recycling of non-packaging material can befound. Here again, acid or anhydride-grafted polyolefins [106,107], polyolefins grafted with glycidylmethacrylate [107–109], or ionomers are used in the literature [110,111].

Also, the compatibilization of packaging plastics other than polyolefins, PET, and PA is describedin the literature [111–115].

Additionally, a method is described in which PP and PET were processed together without acompatibilizer. Microfibrillar composites were instead produced, which means that, in a mixture of twoimmiscible polymers, the polymer with the highest melting point is used as a natural reinforcementelement in the one with the lower melting point. Therefore, a special thermal–mechanical treatmentconsisting of three steps has to be performed: first, the immiscible polymers are melt-blended to createa dispersion of the polymers; in a second step, the filaments are continuously drawn at a certaintemperature; in the third step, the isotropization is conducted. In the case of PET and PP, PET formsmicrofibrils in the PP matrix, whereby improved mechanical properties result. This processing methodcan be seen as an option to utilize PP waste streams containing small amounts of PET [116,117].

4. Summary & Conclusions

Decent sorting is a basic prerequisite for the production of high-quality recycled polymers.According to the current state of the art, plastic and plastic multilayer packaging is not assigned toa special sorting stream. Instead, plastic and plastic multilayer packaging can be found in differentrecycling pathways like films, mixed plastics, or the residue.

The residue and, in some cases, the mixed plastics are utilized directly by incineration. In thecases of mechanical recycling, as it is performed with films and, in some instances, also with mixedplastics, multilayers are usually an impurity and have to be separated from the presorted input in asecond separation step.

The current state of research, in general, describes two general ways to recycle multilayeredpackaging items: the first option is to separate the different multilayer components and make themavailable for recycling in separated recycling streams; the second option is to process the usedcomponents together in one compatibilization step.

Compatibilization is achieved by the addition or generation of suitable molecules that act ascompatibilizers. The methods, which are based on the separation of the different materials of amultilayer, can be subdivided into two strategies: either the determined target polymers are separatedonly by the selective dissolution–reprecipitation method, or the multilayer is delaminated by physical,chemical, or mechanical means (cf. Figure 7). Physical delamination methods in general are based onthe dissolution of either an adhesive or an interlayer to induce delamination and thus the separationof the different layers. As solvents, organic solvents, water and aqueous solutions with a pH valuedifferent from seven were mentioned. Mechanical delamination remains an exception, since, usually,a strong adhesion between the different layers is given. Only one case is known in which weakadhesion allows for the mechanical separation of a multilayered packaging by shredding. The methodsin which the separation of the different layers is induced by a chemical reaction can be subdividedinto methods in which delamination is induced by a reaction at the interphase, and those in whichdelamination is induced by decomposition of an interlayer (e.g., Al, whey-protein, PET).

The technique of compatibilization has the big advantage that it is uncomplicated in performance.However, with regard to both the quality and the costs, the optimal amount of compatibilizer has to beadded, since neither a shortage of compatibilizer, nor an excess amount of compatibilizer is desirable.This means that the composition of the input should be known. Postindustrial waste has a constantand known composition and thus can be recycled by compatibilization.

In the case of commingled postconsumer multilayer packaging waste, a very high degree ofsorting would be necessary to provide a consistent product quality that is usually desired by thepurchaser. However, the composition of postconsumer waste is subject to fluctuations, and no ideal

Recycling 2018, 3, 1 20 of 26

sorting can be guaranteed. Thus, the scenario of commingled postconsumer multilayer packagingwaste by compatibilization does not seem to be realistic.

Recycling 2017, 2, 25 19 of 25

are based on the dissolution of either an adhesive or an interlayer to induce delamination and thus

the separation of the different layers. As solvents, organic solvents, water and aqueous solutions with

a pH value different from seven were mentioned. Mechanical delamination remains an exception,

since, usually, a strong adhesion between the different layers is given. Only one case is known in

which weak adhesion allows for the mechanical separation of a multilayered packaging by

shredding. The methods in which the separation of the different layers is induced by a chemical

reaction can be subdivided into methods in which delamination is induced by a reaction at the

interphase, and those in which delamination is induced by decomposition of an interlayer (e.g., Al,

whey-protein, PET).

Separation of the different components

Combined processing of the different components

Multilayer Recycling

Chemical delamination of the different layers

Physical delamination

Chemical delamination by decomposition of an interlayer

Delamination caused by reactions at the interface

Mechanical delamination

Separation of the different components

Recycling of a target polymer by dissolution and

reprecipitation

Figure 7. Schematic overview of the introduced recycling methods of multilayer packaging.

The technique of compatibilization has the big advantage that it is uncomplicated in

performance. However, with regard to both the quality and the costs, the optimal amount of

compatibilizer has to be added, since neither a shortage of compatibilizer, nor an excess amount of

compatibilizer is desirable. This means that the composition of the input should be known.

Postindustrial waste has a constant and known composition and thus can be recycled by

compatibilization.

In the case of commingled postconsumer multilayer packaging waste, a very high degree of

sorting would be necessary to provide a consistent product quality that is usually desired by the

purchaser. However, the composition of postconsumer waste is subject to fluctuations, and no ideal

sorting can be guaranteed. Thus, the scenario of commingled postconsumer multilayer packaging

waste by compatibilization does not seem to be realistic.

Additionally, compatibilization only adds one additional life cycle to the polymer, and the end

of life treatment of material recycled by compatibilization is incineration.

It is the only method that is successfully established on the market today, e.g., to improve the

quality of slightly contaminated polymers or the recycling of postindustrial packaging waste. Thus,

compatibilization is used, for instance, to handle poorly sorted PE or PP fractions recycled from

packaging waste. Its scope, however, is limited by the described issues.

The method of the selective dissolution and recovery of one target polymer from a mixture of

different packaging and polymer types does not require a high degree of sorting. Here, neither the

input nor the output has to run through a complex sorting process, and the input can thus contain

different multilayer and nonmultilayer packaging systems. The quality of the obtained polymer can

Figure 7. Schematic overview of the introduced recycling methods of multilayer packaging.

Additionally, compatibilization only adds one additional life cycle to the polymer, and the end oflife treatment of material recycled by compatibilization is incineration.

It is the only method that is successfully established on the market today, e.g., to improve thequality of slightly contaminated polymers or the recycling of postindustrial packaging waste. Thus,compatibilization is used, for instance, to handle poorly sorted PE or PP fractions recycled frompackaging waste. Its scope, however, is limited by the described issues.

The method of the selective dissolution and recovery of one target polymer from a mixture ofdifferent packaging and polymer types does not require a high degree of sorting. Here, neither theinput nor the output has to run through a complex sorting process, and the input can thus containdifferent multilayer and nonmultilayer packaging systems. The quality of the obtained polymer canbe high enough to compete with the virgin material and free of additives or other contaminants.In general, however, the recycling of the sorting fractions that are, according to the current situation,utilized by incineration is quite conceivable with this method.

However, one big drawback is that the solvent has to be removed from the polymer solutionsunder expenditure of energy and time. This method is the most efficient if only few polymers make upmajor parts of the total amount. The economic efficiency decides on the number of polymers separatedfrom the polymer mixtures. Thus, one further disadvantage is that, usually, not all components ofmore complex multilayers or mixtures of different multilayer structures are recovered.

Until today, extensive research has been done on the polymer recovery by the dissolution–reprecipitation technique. First industrial pilot plants exist or are currently installed, however,the feasibility in industrial practice and sustainable economic efficiency has not been proven yet.

In contrast to the dissolution–reprecipitation technique, recycling by delamination only dissolvesa minor, if any, component. For this reason, the amount of polymer that will be regained and dried ismuch lower, and, hence, the delamination processes could be more energy-efficient and cost-effective.Additionally, not only the main component, but all components could be regained.

Recycling 2018, 3, 1 21 of 26

The methods that provide the delaminated multilayer components have in common the featurethat the different materials must be separated from each other to be recycled in single material streams.

This separation step is essential for the quality of the recycled polymer. What has to be takeninto account is that, with increasing heterogeneity of the mixture, the complexity of the separationgrows. Therefore, the input should be as defined as possible, since a defined input is a prerequisite fora successful separation process later on. This means that a separate recycling stream for delaminablemultilayers has to be established. This scenario would require the application of new marker-basedsorting technologies.

Currently, however, only delamination methods suitable for particular types of multilayer exist.Waste fractions containing only one multilayer occur at specific productions sites with low amounts,maybe too low to justify an investment in a sophisticated recycling technology. A more general methodto delaminate a broader range of multilayer structures would be desirable, as it would be applicablefor real packaging waste.

In order to shape a future with higher recycling rates and more efficient recycling procedures,innovative techniques and methods have to be established. In addition to the substitution ofmultilayered packaging by monomaterial solutions, innovative recycling methods can be considered.

Common compatibilization does not seem to be a feasible technique for postconsumer packagingwaste, since fluctuations in the composition of the input can be unpredictable and no constant productquality can be guaranteed. The direct addition of compatibilizer into the packaging system, however,might be a strategy to avoid these problems.

Because of its advanced stage of development, the dissolution–reprecipitation method could beavailable for the recycling of existing recycling streams in the near future. It could be an attractivealternative to incineration or to the production of low-quality recycled materials. In the long term,the recycling of multilayer packaging by delamination might also be an attractive option. Here,in contrast to the dissolution–reprecipitation method, a lower amount of energy has to be expendedfor polymer drying. Thus, a systematical delamination procedure could be an ecologically andeconomically more sustainable solution if the separation of the delaminated polymer layers as well asa selective sorting of recyclable multilayer packaging items are possible.

Acknowledgments: This work was supported by the Industry Association for Food Technology andPackaging (IVLV).

Author Contributions: Katharina Kaiser wrote the manuscript and was involved in the revision and completionof the work. Markus Schmid and Martin Schlummer were responsible for the overall outline and contributed toselected sections as well as to the revision and editing of the manuscript.

Conflicts of Interest: The authors declare no conflict of interest.

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